vlan: Ignore multiple NEWLINK messages

vlan: Ignore multiple NEWLINK messages

hostapd receives NEWLINK messages multiple times and thus does
configuration of the the vlan interface multiple times.

This is not required and leads to the following during cleanup in
test pmksa_cache_preauth_vlan_used:
 1. run-test.py does: brctl delif brvlan1 wlan3.1
 2. hostapd processes NEWLINK and does: brctl addif brvlan1 wlan3.1
 3. run-test.py does: brctl delbr brvlan1
    -> fails as wlan3.1 is still in the bridge

This patch fixes this by ignoring repeated NEWLINK messages.

Signed-off-by: Michael Braun <michael-dev@fami-braun.de>